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[Koichiro Yaji](https://orcid.org/0000-0002-0721-1316), [Shunsuke Tsuda](https://orcid.org/0000-0001-6209-8048)

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[Development of a Photoemission Microscopy Apparatus Using a Vacuum Ultraviolet Laser](https://mdr.nims.go.jp/datasets/50cd948a-a6f7-4146-9a46-30bf8518d4fa)

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e-Journal of Surface Science and Nanotechnology 22, 46–52 (2024)Development of a PhotoemissionMicroscopy ApparatusUsing a Vacuum Ultraviolet LaserKoichiro Yaji,† Shunsuke TsudaCenter for Basic Research on Materials, National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki, 305-0003, Japan† Corresponding author: yaji.koichiro@nims.go.jpReceived: 7 August, 2023; Accepted: 6 September, 2023; J-STAGE Advance Publication: 12 October, 2023; Published: 12 October, 2023We report on a photoemission microscopy apparatus using a10.9-eV laser developed at the National Institute for MaterialsScience (NIMS). Our spectrometer realizes photoemission spec-troscopy with a high spatial resolution by combining an imagingdouble energy analyzer with the electronic lens system ofphotoelectron emission microscopy. Energy-filtered photoelec-tron imaging is available in both real and momentum spaces.The spatial resolution in the real space mode is ~30 nm. Weshow energy-filtered photoelectron images of a silver grid-pat-terned sample in real space and the band mapping of Au(111) inmomentum space to demonstrate the performance of our spec-trometer.straight channel PEEM lens laserIDEAphotoelectronsamplespin channelexit lensx-distance (μm)y-distance (μm)kx (Å-1)k y (Å-1)Keywords Photoemission spectroscopy; Momentum microscope; Ultraviolet laser; Electronic propertyI. INTRODUCTIONElectronic states near the Fermi level play an importantrole in determining the electrical and magnetic properties ofmaterials. Thus, visualization of the behavior of electrons inmaterials is quite useful for designing and developing func-tional devices. Electronic states in materials are described bythree quantum numbers: energy, momentum, and spin. An-gle-resolved photoemission spectroscopy (ARPES) enablesus to observe the energy dispersion of the band [1]. Spin-resolved photoemission spectroscopy provides informationon the spin-polarized band structure [2]. Thus, the photo-emission spectroscopy of the valence band is a powerfultechnique for characterizing materials. ARPES has played acentral role in the study of electronic states for superconduc-tors, topological materials, low-dimensional materials, etc.[3] Furthermore, in the last decade, there has been a tremen-dous movement toward probing electronic states in a sub-micrometer region [4]. This is due to the demands of eluci-dating the local electronic states of materials for developinginnovative devices. For instance, the studies of superconduct-ing states and topological phases in novel quantum materialsrequire the characterization of electronic bands below thesub-micrometer region [5, 6]. Conventional photoemissionspectroscopy has a spatial resolution of several tens of µm ormore, making it challenging to obtain information on thelocal electronic states in a few-micrometer to sub-micrometerregion.In this context, the development of photoemission micros-copy apparatuses for investigating the electronic state in thesub-micrometer region has recently been accelerated. Syn-chrotron radiation facilities are leading the development ofsuch apparatuses: currently, the photoemission microscopymachines for analyzing the valence electrons in the severalhundred-nanometer ranges are operated at several synchro-tron radiation facilities [7–10]. In addition, a spatial resolu-tion of 70 nm was achieved by core-level photoemissionmicroscopy with an external voltage applied at BL07LSUat SPring-8, resulting in a deep understanding of the elec-tronic structure of materials in their operating environment[11]. In these techniques, synchrotron radiation is focusedusing optical focusing mirrors and/or Fresnel zone plates, andthe sample is scanned in the lateral direction to obtain thespatially resolved electronic state.Technical Notee-J. Surf. Sci. Nanotechnol. 22, 46–52 (2024) | DOI: 10.1380/ejssnt.2023-066 46mailto:yaji.koichiro@nims.go.jphttps://doi.org/10.1380/ejssnt.2023-066Recently, an imaging-type spectrometer for photoemissionmicroscopy, which utilizes a different mechanism from thescanning type in principle, the so-called momentum micro-scope, has made significant progress [12–20]. This spectrom-eter realizes the high spatial resolution by employing theelectronic lens system based on photoelectron emission mi-croscopy (PEEM) in the lens section of the photoelectronanalyzer. The photoelectrons emitted from a sample aretypically accelerated to energies of 10–20 keV by the highelectrostatic field between the first electronic lens of theanalyzer and the sample. Thus, high spatial resolution canbe achieved even in low-energy photoexcitation, such as anultraviolet laser, not limited by the diffraction limit of theincident light. The spatial resolution of 2.6 nm has beenachieved in PEEM using a 4.66-eV (λ = 266 nm) laser [21].We note here that the spatial resolution of the PEEM isrestricted by the intrinsic vibration originating from the in-strument itself in most cases. Thus, this imaging-type spec-trometer enables photoemission microscopy with lab-basedlight sources, such as a laser, a mercury (Hg) lamp, a helium(He) lamp, and an X-ray tube, without synchrotron radiation.Besides, thanks to the PEEM lens system, one can quicklyswitch between real-space and momentum-space measure-ments. These technologies are employed in Nano-ESCA[12–15] and Momentum Microscopy [16–20]. These spec-trometers are capable of electronic state measurements inthree measurement modes: (1) non-energy filter PEEMmode, where the magnified images of the surface can beobserved, (2) energy-filtered photoelectron imaging mode inreal space, where the area-selected imaging from the workfunction cut-off to the Fermi level cut-off energies is avail-able, (3) energy-filtered photoelectron imaging mode in mo-mentum space, where the band mapping in a selected areawith an aperture is available. Photoemission microscopyapparatuses using Nano-ESCA or Momentum Microscopyhave been developed at several synchrotron radiation facili-ties [14, 16, 20]. The time-resolved measurements using apulsed laser [22, 23] and the chemical analysis using abrilliant X-ray [24] have been performed in the laboratory-based system.In this article, we report on a photoemission microscopyapparatus using a 10.9-eV laser recently developed at theNational Institute for Materials Science (NIMS). Our ma-chine is equipped with Nano-ESCA with a state-of-the-artImaging Double Energy Analyzer (IDEA). This allows oneto obtain energy-filtered photoelectron images in real andmomentum spaces. The spectrometer achieves an energyresolution of 24meV, a special resolution of ~30 nm in a realspace mode, and a wave number (k) resolution of 0.02Å−1 ina momentum space mode. We show the band mapping ofAu(111) as a demonstration of ARPES.II. INSTRUMENTSA. Overview of the apparatusFigure 1 shows an overview of the photoemission micros-copy apparatus developed at NIMS. Our apparatus consistsof three ultrahigh vacuum (UHV) chambers: an analysischamber, an intermediate chamber, and a sample preparationchamber. These chambers are connected via UHV gatevalves. The base pressure of the analysis chamber is keptbelow 1.5 × 10−8 Pa. The analysis chamber is equipped witha vacuum ultraviolet (VUV) laser, a He discharge lamp, and aHg lamp as excitation light sources. The VUV laser isincident at an angle of 65° with respect to the sample surfacenormal. The incident plane of the VUV laser is parallel to thex–z plane. Photoelectrons are analyzed by Nano-ESCA madeby Focus GmbH. The typical distance between the sampleand the photoelectron analyzer is approximately 2.5mm. Thesample manipulator is driven by a stepping motor in 6 axes:x, y, z, θx, θy, and ϕ. Samples can be cooled below 20K usingliquid helium. One can access from the intermediate chamberto the analysis and sample preparation chambers using UHVtransfer rods. Six samples can be stocked in the intermediatechamber. A load lock for sample entry and a UHV suitcasemade by Ferrovac AG can be connected to the intermediatechamber. The sample preparation chamber is equipped withan Ar+ sputter gun and a low-energy electron diffraction(LEED) instrument. Three ports are provided for evaporatormounting. Samples can be heated by both direct currentheating and electron bombardment heating. The sample ma-nipulators in the sample preparation chamber can be cooledwith liquid nitrogen and liquid helium.B. 10.9-eV laserThe electrons in the sample are excited by 10.9-eV(113.8 nm wavelength) photons, yielded by the 9th harmonicof a Yb fiber laser with the wavelength of 1024 nm (OXIDEUV-3), where the basic idea of the laser system has beenreported in the reference [25]. The repetition rate is 50MHz,and the pulse duration is less than 20 ps. Our laser systemoffers linear horizontally and vertically polarized lights andleft- and right-circularly polarized lights with individuallycontrollable half- and quarter-wave plates.Using the high-brilliant laser provides a significant advant-age in drastically improving the energy and momentumresolutions of photoemission spectroscopy [26–28]. On theother hand, the photon energy of the laser in most laser-ARPES is 6–7 eV, resulting in the observable wavenumber kbelow 0.6–0.8Å−1. The 6–7 eV lasers are significantly help-ful in the electronic structure measurements near the Γ point.In contrast, photoexcitation with a 10.9-eV laser extends theobservable wavenumber to 1.3Å−1. Thus, the electronicstates from the Γ point to the edge of the first Brillouin zonefor many materials are observable.C. Photoelectron analyzerOur Nano-ESCA comprises a PEEM lens system, IDEA, adelay line detector (DLD), and a two-dimensional (2D)imaging unit with multi-channel plates and a CMOS camera[Figure 2(a)]. The PEEM lens system allows photoelectronimaging in real (x–y) and momentum (kx–ky) spaces. Thestraight channel without IDEA provides standard PEEMTechnical Notee-J. Surf. Sci. Nanotechnol. 22, 46–52 (2024) | DOI: 10.1380/ejssnt.2023-066 47https://doi.org/10.1380/ejssnt.2023-066images. The straight channel is equipped with the DLD on alinear shifter. Thus, inserting the DLD into the PEEM col-umn realizes time-of-flight energy analysis in the straightchannel. Photoemission spectroscopy using this DLD willbe described elsewhere. In this article, we focus on theenergy analysis with the IDEA. Photoelectrons travelingthrough the PEEM lens are energy-filtered by the IDEA.Here, one can select five different pass energies: 12.5, 25,50, 100, and 200 eV, and five slit widths: 0.2, 0.5, 1, 2, and8mm. The spin analysis is possible by inserting a spindetector in the exit lens section of the IDEA. The maximummeasurable wave number of Nano-ESCA is kx,y ~ ±3.0Å−1.Thus, the Nano-ESCA can simultaneously analyze the photo-electrons emitted in the range of 2π sr with photoelectronkinetic energies below ~34 eV. In the measurement, themagnification of the real and momentum spaces can beoptimized according to the electronic state to be observed.Furthermore, a spot selector (continuously variable iris aper-ture) can select an area of interest down to sub-micrometers.D. Laser spot sizeThe PEEM image of a silver (Ag) grid-patterned sampleon a silicon (Si) substrate measured with a field of view(FoV) of 711µm is shown in Figure 2(b). The thickness of6-axis manipulatoranalysischamberintermediate chamberload lockUHV suitcaseDLD on linear shifterMBE chamberxyzxlaserHe lampanalysischamberTop viewSide viewHg lampHe lamplaserxyzy(a)(b)HNano-ESCANano-ESCA2Ddetector2Ddetector2DdetectorIDEAIDEAFigure 1: Overview of the apparatus. The top and side views are shown in (a) and (b), respectively.Technical Notee-J. Surf. Sci. Nanotechnol. 22, 46–52 (2024) | DOI: 10.1380/ejssnt.2023-066 48https://doi.org/10.1380/ejssnt.2023-066Ag is roughly 40 nm. The sample is the same as shown inFigure 3, but the grid pattern is not visible in Figure 2(b)because the grid size is much smaller than the FoV. From thisphotoelectron intensity mapping, the full-width half maxi-mum (FWHM) of the laser spot on the sample is estimated tobe 320 µm in the horizontal direction and 130 µm in thevertical direction. Here, the actual beam size in the horizontaldirection is 140 µm since the incident angle of the laser to thesample normal is 65°. Therefore, the laser spot shape isalmost circular, while the horizontal footprint on the sampleis elongated due to the oblique incidence. Stray light is found150 µm below the main spot.III. PERFORMANCEA. Spatial resolutionFigure 3(a, b) shows a photoelectron image with 104 and10.7 µm FoVs from the Ag grid pattern on the Si substrate,respectively. Each patch is an 8 × 8 µm2 square separated bytrenches with 2 µm widths. We used the Hg lamp as theexcitation light source. We detected the photoelectrons withalmost zero kinetic energy, meaning that the contrast in theDLD(removable)straight channel PEEM lenslaser2DimagingunitIDEAphotoelectronsampleImaging-type spin detectoravailable spin channelx-distance (μm)0 200 400 600200400600y-distance (μm)0FWHM: 320 μmFWHM: 130 μm(a)(b)exit lens2Dimagingunit2Dimagingunitxyz Figure 2: (a) Schematic drawing of Nano-ESCA. Bold green linesrepresent the photoelectron trajectory. (b) Photoelectron intensitymapping from the Ag grid-patterned sample on the Si substrate inthe real space with a FoVof 711 µm. The 10.9-eV laser is used as anexcitation light.x-distance (μm)0 10020 40 60 8010020406080y-distance (μm)x-distance (μm)0 102 4 6 80102468y-distance (μm)x-distance (μm)y-distance (μm)2.3 7.52.5 7.72.7 7.9IntensityIntensity(a)(c) (d)(b)FWHM:27 nmFWHM:36 nm0#1#2#1 #2swept direction of the exposure lineFigure 3: Photoelectron intensity mapping of the square grid pat-tern of Ag on the Si substrate in real space with the FoV of 104µm(a) and 10.7 µm (b), where the photoelectron near the cut-off energyby the work function is detected. Photoelectrons were excited by theHg lamp (hν = 5.2 eV). The exposure line of the CMOS camera isswept from the center to outward in the y-direction. (c, d) Photo-electron intensity profiles along #1 and #2 shown in (b). Each datapoint is obtained by the summation of the photoelectron intensitywithin a window given by the line widths of #1 and #2. Circlesymbols represent the experimental data. Solid curves are fittingresults with the step function convoluted by the Gaussian.Technical Notee-J. Surf. Sci. Nanotechnol. 22, 46–52 (2024) | DOI: 10.1380/ejssnt.2023-066 49https://doi.org/10.1380/ejssnt.2023-066image reflects the difference in photoelectron yield resultingfrom the work function difference between the square patchesand the trenches. The measurement was performed at roomtemperature. We set to the pass energy of 100 eV for IDEA.The exposure time of the CMOS camera was 10ms.The photoelectron intensity profiles at the edges of thesquare patches along #1 and #2 are shown in Figure 3(c, d),respectively. We evaluate the spatial resolution from theseintensity profiles in the real space mode. The intensity pro-files are fitted by a step function convoluted by a Gaussian,where linear functions with different slopes on the trench andthe square patch are used as a background. Here, we definedthe spatial resolution by the FWHM of the Gaussian. As aresult, the spatial resolution at #1 (#2) is estimated to be27 nm (36 nm). We find that the spatial resolution in the y-direction is 30% better than that in the x-direction. This resultmight be caused by the image acquisition method of theCMOS camera. The image readout of the CMOS cameraadopts a rolling shutter method, where an exposure line alongthe x-axis with one-pixel width is swept in the y-direction.Thus, the intensity data in the exposure line is acquiredsimultaneously, while the acquisition time difference occursin the different exposure lines. Each data point of the inten-sity profile of #1 (#2) is obtained by the summation of thephotoelectron intensity within a 750 nm window, correspond-ing to 74 pixels, in the x-(y-)direction. Therefore, the inten-sity profile of #2 is more affected by the vibration of theinstruments than that of #1.B. Energy resolutionThe energy resolution of our spectrometer is evaluatedwith Au(111). The 10.9-eV laser was used as an excitationsource. The measurement was performed with the momen-tum space mode, and the IDEA was used for the energyfiltering. The pass energy of the IDEAwas set to 12.5 eV andthe slit width to 0.5mm. The sample temperature was kept at50K. Plots in Figure 4 are obtained by integrating the photo-electron intensity in the wavenumber region where the bulkband of Au(111) appears. The intensity plots are fitted with aFermi-Dirac distribution function convoluted by the Gauss-ian. From the fitting, the energy resolution is estimated to be24meV.IV. DEMONSTRATIONAs a demonstration of ARPES measurement with oursystem, we measured the electronic band structure ofAu(111). It is well known that a Shockley-type surface stateappears around �� in the surface Brillouin zone of Au(111).The surface state exhibits a parabolic energy dispersion witha bottom at a binding energy of 0.48 eV, and the Fermisurface is circular [29]. Besides, the spin degeneracy of thesurface state is lifted due to the strong spin-orbit interaction,resulting in the Rashba-type spin-split bands [30]. Spin polar-ization of the Au(111) surface state has been investigated indetail by SARPES [31]. In addition to Au(111), the spinpolarizations of the Shockley-type surface electronic states ofCu(111) and Ag(111) have also been revealed by high-reso-lution SARPES [32].A clean surface of Au(111) was obtained by cycles of Ar+sputtering and annealing at 550°C. The clean surface wasconfirmed by observing sharp (1 × 1) spots and additionalsatellite spots attributed to the herringbone superstructure onthe Au(111) surface by LEED. Photoelectrons were excitedby the 10.9-eV laser with a p-polarization, where the electric-field vector of the laser is parallel to the light incident plane.We used the IDEA for the energy analysis of the photo-electron, where the pass energy was 25 eV, and the slit widthwas 0.5mm. The FoV in the real space was set to 80µm. Thesample temperature was kept at 30K during the measure-ments.Figure 5(a, b) displays an E–kx–ky cube from the lowestcut-off to the highest cut-off energies (i.e., from zero kineticenergy to the Fermi level) and an E–kx image obtained byslicing the E–kx–ky cube at ky = 0, respectively. In our spec-trometer, the energy dispersion of the band is obtained as athree-dimensional cube: energy (E), x wavenumber (kx), andy wavenumber (ky). Therefore, the band structure of any E–k∥cuts of interest can be obtained, as demonstrated in Figure5(b). A parabolic photoemission horizon with an energybottom of EB = 5.5 eV is identified. The Shockley-type sur-face state appears near the Fermi level around ��, whichagrees with the previous report [29]. Several bands attributedto the bulk states are observed in the deeper binding energyside.The constant energy ARPES intensity image at EB =5.0 eV is shown in Figure 5(c). The momentum (k) resolutionis evaluated from the intensity profile of the edge of thephotoemission horizon [Figure 5(d)]. In the fitting procedure,we used the step function convoluted by the Gaussian, as inthe case of the spatial-resolution estimation shown in Figure3(c, d). As a result, the k resolution is estimated to be0.021Å−1.Figure 5(e) shows the Fermi surface image of Au(111)recorded with the same experimental conditions as in Fig-ure 5(a), but only with improved statistics. A circular-shapedFermi surface centered at �� is observed, attributed to theBinding energy (meV)04080 -80-40Intensity (arb. units)Figure 4: Photoelectron intensity from Au(111) at the Fermi level.The IDEA is used for energy filtering. Square symbols represent theexperimental result. The solid curve is a fitting result with theFermi-Dirac distribution function convoluted by the Gaussian.Technical Notee-J. Surf. Sci. Nanotechnol. 22, 46–52 (2024) | DOI: 10.1380/ejssnt.2023-066 50https://doi.org/10.1380/ejssnt.2023-066Shockley-type surface state. In our result, however, no appa-rent band splitting has been visible. According to the pre-vious study [29, 31], the size of the spin splitting of thesurface state for Au(111) is 0.02Å−1, which is comparable toor slightly smaller than the k resolution of our spectrometer.Therefore, we could not observe a clear band splitting due tothe lack of the k resolution.V. SUMMARYA photoemission microscopy apparatus with Nano-ESCAusing a 10.9-eV laser has been developed at NIMS. Ourapparatus enables energy-filtered photoelectron imaging inboth real and momentum spaces. The energy resolution ofthe spectrometer has been estimated to be 24meV. In the real(momentum) space mode, the spatial (k) resolution has been30 nm (0.021Å−1). We have performed the ARPES measure-ments of Au(111) to demonstrate the capability of our appa-ratus. The high spatial resolution of the machine paves theway for new opportunities, particularly in fields such assubmicrometer-scale materials, polycrystals, andcombinato-rial materials that have yet to be the target of the conventionalphotoemission spectroscopy.AcknowledgmentsThe authors thank Nils Weber for technical support in developingthe spectrometer. The present work was partially supported by theJapan Society for the Promotion of Science KAKENHI (Grant Nos.JP21K04633 and JP22H01761), the Innovative Science and Tech-nology Initiative for Security Grant Number JPJ004596, ATLA,Japan, and Iketani Science and Technology Foundation.References[1] S. Hüfner, Photoelectron Spectroscopy: Principles andApplications (Springer-Verlag, Berlin, Heidelberg, 2003) Chap. 7.[2] P. D. Johnson, Rep. Prog. Phys. 60, 1217 (1997).[3] J. A. Sobota, Y. He, and Z.-X. Shen, Rev. Mod. Phys. 93,025006 (2021).[4] L. Mino, E. Borfecchia, J. Segura-Ruiz, C. Giannini, G.Martinez-Criado, and C. Lamberti, Rev. Mod. 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